The functions of disc brakes for motorcycles, motorcars, bicycles, and the like are to slow the rotation of wheels by the friction between brake disks and brake pads to control the speed of vehicles. Therefore, the brake disks need to have proper hardness. There is a problem in that a low hardness of the brake disks reduces the braking performance and accelerates abrasion of disk because of the friction with brake pads and an extreme hardness thereof causes brake squeal. It is recommended that the proper hardness of the brake disks be within a range from 32 to 38 HRC. The term “HRC” used herein means the Rockwell C hardness specified in JIS Z 2245.
A material conventionally used for the brake disks is martensitic stainless steel in view of hardness and corrosion resistance. In the past, martensitic stainless steel, such as SUS 420J2 (JIS Z 4304), having a high carbon content was used for the disks after quenching and tempering treatment. Since the workload of tempering treatment is large, low-carbon martensitic stainless steel has been recently used for the brake disks as disclosed in Japanese Unexamined Patent Application Publication No. 57-198249 or 60-106951 because this steel can be used directly after quenching treatment.
In view of global environmental conservation, recent motorcycles and motorcars need to have high fuel efficiency. A reduction in vehicle weight is effective in achieving high fuel efficiency; hence, lightweight vehicles are demanded. Even disc brakes, which are a part of brake mechanism (or brake system), are no exception. For the weight reduction of vehicles, compact or low-thickness (thin) brake disks are being experimentally produced.
Compact or thin brake disks have low heat capacity. Hence, the temperature of the disks is greatly increased by friction heat during braking. That is, the compact or thin brake disks are probably heated to 600° C. or higher during braking. Brake disks made of conventional materials are probably reduced in durability by temper-softening. Therefore, brake disks that have high temper softening resistance or are excellent in temper softening resistance are demanded.
To cope with such a demand, the following sheet has been proposed as disclosed in Japanese Unexamined Patent Application Publication No. 2002-146489: a low-carbon martensitic stainless steel sheet which contains a proper amount of one or more of Ti, Nb, V, and Zr and which can be effectively prevented from being softened by heating during the use of a disc brake.
Japanese Patent 3315974 (Japanese Unexamined Patent Application Publication No. 2001-220654) discloses a stainless steel for disc brakes. The stainless steel contains a proper amount of Nb or a proper amount of Nb and Ti, V, and/or B and therefore can be prevented from being temper-softened.
Japanese Unexamined Patent Application Publication No. 2002-121656 discloses low-cost steel for disc brake rotors. The GP value of this steel is adjusted to 50(%) or more and this steel contains a proper amount of Nb and/or V, the GP value being determined by a function of the content of C, N, Ni, Cu, Mn, Cr, Si, Mo, V, Ti, and Al in this steel. This steel is hardly deteriorated by heating during operation.
In the technique disclosed in Japanese Unexamined Patent Application Publication No. 2002-146489, Japanese Patent 3315974, or Japanese Unexamined Patent Application Publication No. 2002-121656, there is a problem in that a relatively large amount of high-cost alloying elements need to be used and therefore the cost of manufacturing a disc brake is high. Furthermore, there is a problem in that the steel sheet or steel is significantly reduced in hardness after being held at 600° C. for a long time (about one hour). Disc brakes are critical components for driving safety and therefore need to have high toughness sufficient to prevent embrittlement cracking.
It could therefore be helpful to provide a brake disk which is effective in solving problems caused by the conventional techniques, which has proper hardness after quenching, and excellent temper softening resistance and toughness.